In late December 2015 the Russian Defense Ministry finally agreed with a member of the United Aircraft Corporation (OAK) by "dry" five-year contract to supply 50 Su-35, which comes into force in the coming year, told "Vedomosti" two management companies KLA and confirmed the person close to the Defense Ministry. The amount of the contract, according to one of the managers, more than 60 billion rubles. The plan was signed in August 2015 during the MAKS-2015, however, due to the failure of the three-year budget planning difficulties with the definition of the parameters of the contract, said the source "Vedomosti". As a result, the contract was finalized and approved only after President Vladimir Putin signed the law on the federal budget for 2016, he said. However, in previous years, major contracts for the supply of military equipment to the Ministry of Defence has repeatedly concluded in the last days of December - "under the tree."This is interesting. The Russians have the PAK-FA in development, yet instead of tilting entirely toward their next gen stealth fighter they're continuing to buy SU-35's.
What does this tell us about the F-35?
Consider that if the Chinese were able to hack our data then the Russians were there before they were to take a peek under the hood of that airplane.
Buying more SU-35's tells me that it doesn't scare them one bit. As a matter of fact it also indicates that they're looking at technological parity and will seek to gain numerical superiority.
My guess is that they think that can beat the F-35 in its go to war configuration (block 4) with the SU-35 and they'll use the PAK-FA to go head to head with the F-22.
Why is this important? Because it will affect the number of PAK-FA's that they need to build. If the SU-35 is superior to our legacy fighters (on this I have no idea) and will at least equal the F-35 then they don't need to spend as much on the PAK-FA and can put fewer into service than they originally projected. Which means that they will flood the zone with new built SU-35s.
If even half of my wild ass guess is right then the West has already lost air superiority. It would also explain the wild and wooly pronouncements from Air Force officials regarding the Russian threat.